1. Field of the Invention
The application relates to a method used in a communication device in a wireless communication system, and more particularly, to a method of monitoring search space of enhanced downlink control channel in orthogonal frequency-division multiple access system.
2. Description of the Prior Art
Physical downlink control channel (PDCCH) is necessary for 3rd Generation Partnership Project (3GPP) long term evolution (LTE) release 8-10 system to maintain communication via control information. The PDCCH in a LTE system carries user equipment (UE) specific scheduling assignments for Downlink (DL) resource allocation, Uplink (UL) grants, Physical Random Access Channel (PRACH) responses, UL power control commands, and common scheduling assignments for signaling messages (such as system information, paging, etc.). PDCCH is built based on Orthogonal Frequency-Division Multiple Access (OFDMA) system.
As technology advances, PDCCH is no longer suitable for further complicated network deployments and transmission schemes such as heterogeneous network and coordinated multipoint transmission/reception (CoMP) for its inability of frequency domain inter-cell interference cancellation (FDM-ICIC) and lack of finer granularity and also lack of enough control channel capacity. As a result, an enhanced physical downlink control channel (EPDCCH) is proposed to provide a more flexible and robust control channel under the various scenarios for 3GPP release 11.
It is agreed in 3GPP standard that a minimum aggregation level monitored by a UE could consist of much more or less resource elements (REs) compared to the legacy PDCCH where a minimum aggregation level monitored by the UE consists of fixed 36 REs equivalent to one control channel element (CCE). Wherein, RE is a minimum resource unit in LTE, indicated by one OFDM symbol in time domain and one subcarrier in frequency domain.
Moreover, it is agreed in 3GPP standard that one physical resource block (PRB) pair has a fixed number of enhanced CCEs (ECCEs) for the EPDCCH. The fixed number of ECCEs in a PRB pair can be 2 or 4 depending on a cyclic prefix (CP) length and the type of subframe. Available REs for EPDCCH transmission in one ECCE can vary a lot in different cases depending on a presence of other signals like cell-specific reference signals (CRS) and legacy PDCCH.
On the other hand, the legacy PDCCH aggregation levels monitored by a UE are 1, 2, 4 and 8 CCEs. Unlike PDCCH, there are two different types of EPDCCH: distributed type and localized type. The distributed type EPDCCH aggregation levels monitored by the UE can be 1, 2, 4, 8 and 16 ECCEs. Furthermore, the distributed type EPDCCH aggregation levels monitored by the UE will be 2, 4, 8, 16 and 32 ECCEs if a number of available REs in a PRB pair is below a certain threshold X, e.g. X=104. For localized type EPDCCH, in which aggregation levels monitored by the UE will be 1, 2, 4 and 8 ECCEs or 2, 4, 8 and 16 ECCEs similarly depending on the threshold X. Specifically, if the number of available REs in a PRB pair is smaller than the threshold X, the monitoring aggregation levels should have twice value (e.g. {2, 4, 8, 16} aggregation levels) than the other cases where the number of available REs in a physical resource block (PRB) pair is larger than the threshold value (e.g. {1, 2, 4, 8} aggregation levels).
However, the applicant notices that a problem arises if the threshold X is fixed (e.g. 104). More specifically, if the threshold X is set too large, a resource waste problem may occur. Otherwise, a reliability of the EPDCCH could be affected. Besides, if the threshold X is fixed, it is likely that some large DCI (e.g. DCI with format 2 or DCI with format 2C) will suffer from not only worse performance, and even become “non-decodable” if a code rate of the large DCI is greater than 1. Specifically, assuming there are 4 ECCEs and 104 available REs in one PRB pair. Then, one ECCE includes 26 REs in average and contains 52 encoded bits (assuming QKSP is used). However, the DCI with format 2C requires 58 bits in general and at least 29 REs should be allocated in one PRB, therefore, the code rate of the DCI with format 2C is 58/52>=1. Namely, part of information of the DCI with format 2C could be truncated after encoding and the DCI with format 2C becomes non-decodable.
Since the UE usually performs blind decoding on all monitoring DCI decoding candidates in the search space of the EPDCCH no matter the monitoring DCI candidate is decodable or non-decodable, in such situation, it is undesirable to have the UE monitoring a non-decodable DCI, which not only increases a blocking rate due to waste of DCI decoding candidates but also wastes power.